He joined the Communist Party in 1938, becoming an organizer for the Durban and District branch. In 1947, he withdrew from active politics in order to concentrate on his legal practice, but he did participate in the Durban branch of the COD in the 1950s.

In 1950, following the Suppression of Communism Act, the South African Communist Party (SACP) decided to disband (though an underground SACP was soon set up). Arenstein worked closely with Chief Albert Luthuli and Chief Mangosuthu Buthelezi. Buthelezi had been expelled from Fort Hare University for his African National Congress (ANC) activities and had come to work at the Durban Commissioner’s Court, the administrative experience being thought useful to the future chief. Arenstein and Buthelezi became close friends. Buthelezi became one of Rowley’s articled clerks. When the chieftaincy of the Buthelezi clan was offered to him, Rowley helped Buthelezi defeat an early challenge to his chieftaincy and became his legal adviser.

Arenstein was first barred from political activities in 1953 when he banned. During this period he was active in organizing opposition to the new laws enforcing apartheid and in establishing labour unions in Durban.Arenstein suffered the longest period of banning (33 years) in South African history and endured the longest house arrest (18 years), with his wife Jackie not far behind: she was house-arrested for six years, banned for 19. His wife, Jacqueline Arenstein, a journalist, was a defendant in the 1956 Treason Trial. Though banned, he continued to defend persons accused of political offences.

Among the ANC activists with whom Buthelezi mingled at the Arenstein house were Nelson Mandela and Walter Sisulu – who always visited Rowley when they came down to Durban to visit Luthuli. Rowley found himself becoming increasingly critical of the autocratic style of the Johannesburg-based Communists who ran the underground SACP and, through it, the ANC.

In 1960 with the Pondoland insurrection against the Government-imposed Bantustan policy, the Pondos were fiercely suppressed and turned to the Arenstein for help. At least 11 people were killed and 6o wounded. Four months after the dead were buried; their remains were exhumed at his insistence after he challenged police claims that fewer than 11 people were killed. Arenstein was barred from leaving the magisterial district of Durban on 1 October 1960 and subsequently could not represent his clients.

He had to go to Pondoland to defend them– many of the Eastern Cape Communists had been detained. When he got back to Durban, a delegation of Pondo leaders came to see him requesting that he facilitates a purchase of guns. Arenstein was able to convince them otherwise and dissuade them from embarking on a violent course. In 1961, he led the legal fight for the release of Anderson Ganyile and other leaders of the Pondoland revolt who had been seized in Lesotho by the South African Police (SAP).

He was vociferous in opposing the move to the armed struggle, predicting that it would bring catastrophe to both the SACP and ANC. During this period, Arenstein claims that he resigned from the Party, although the party insisted that he was expelled.

Arenstein was detained without trial in 1964, and went on hunger strike and was released. When Bram Fischer went underground in 1965, one of the first things he did was to re-establish links with Arenstein, in Natal and Fred Carneson in Cape Town. In 1966 he was sentenced to four years’ jail under the Suppression of Communism Act for furthering the aims of Communism. The prosecution failed to pin a charge of belonging to the SACP. In jail, he developed a strong friendship with the SACP leader Bram Fischer whom he held in very high esteem.

In 1970, Arenstein emerged from jail to find that the ANC had been so utterly smashed that it had effectively ceased to exist within the country. When the Government offered Buthelezi the post of Chief Minister of KwaZulu he sought Arenstein’s advice, and that of the exiled ANC. Both agreed that he should accept– but refuse to take independence.

In 1971, Arenstein remained struck off the official list of attorneys – a punitive measure by the Government which had been in force for twenty years – and was forced to practise from modest offices disguised as a ‘business adviser and consultant’. Although he remained banned he assisted the Defence in the Pietermaritzburg trial of 13, charged under the Terrorism Act. He was later banned from practicing law and placed under house arrest in Durban. He spent the early seventies as an adviser to trade unionists when moves were being made to build up a black labour organisation. He was also a legal advisor to the Inkatha Freedom Party (IFP).

In 1988, despite the vehement protests of the Government, Buthelezi nominated Arenstein as one of his negotiating committee with Pretoria. Thus, Arenstein also served as a legal advisor for the Inkatha Freedom Party.

When Winnie Mandela was denounced for her association with the Mandela United Football Club, Arenstein offered to help her, an offer which she accepted. After Mandela was released Arenstein phoned him and told him that the political violence between the ANC and Inkatha in Natal had to be stopped, and that to achieve this he had to meet Buthelezi. Mandela agreed and told him to arrange such a meeting. The Inkatha stronghold of Taylor’s Halt was chosen as a venue. However, the meeting did not take place but eventually, Mandela and Buthelezi met.

The Arensteins had two children and lived in Durban. Arenstein died in 1996.